This invention relates to bidirectional hot melt ink jet printing systems and, more particularly, to a new and improved hot melt ink jet printing system which provides improved image quality.
In conventional ink jet printing systems, a reciprocating ink jet head having an array of ink jet orifices projects corresponding arrays of ink drops onto a substrate and, for bidirectional printing, the ink drops are applied during each pass of the ink jet head during its reciprocating motion in opposite directions adjacent to the record medium. When a bidirectional ink jet printing system is arranged to provide hot melt ink printing, certain problems may arise which result in degradation of the quality of the image produced by the printing. For example, as described in the copending application Ser. No. 07/359,736, banding may result from variations caused by bidirectional printing and this can be made less apparent by interlaced bidirectional printing. On the other hand, where two adjacent hot melt ink lines are printed in succession during opposite successive motions of a printhead, the first line may not be fully solidified before the second line is printed. If the successive adjacent lines are laid down in two different colors, the inks may coalesce along the border between the lines, producing an undesired line of the composite color.
Even if there is no undesirable mixing of colors, the heat transfer resulting from application of the first line increases the temperature of the adjacent substrate region. As a result, if the line applied during the next scan of the ink jet head is immediately adjacent to the preceding line, the increased substrate temperature in the region where the second line is applied may cause the ink in that line to spread to a greater extent, producing a different image quality.
Furthermore, when two primary colors are overprinted to produce a composite color, the first color often spreads on the substrate to a greater extent than the second color spreads on the first color, so that each drop of the second color ink is surrounded by a marginal region in which only the first color has been printed. This causes the hue of the composite color to be shifted toward that of the first color printed. When different adjacent regions made up of the same composite color are printed by application of the primary colors in reverse order, which may occur during bidirectional color printing, the hues of the composite color in the adjacent regions will be shifted toward that of the primary color first applied, so that regions in which the composite color is applied in opposite directions will show color banding. In addition, the second ink to be applied in the same region will not usually flow into the substrate, which may result in lower adhesion, leading to chipping or flaking of the second ink from the image.